This invention pertains to a method with which biomedical devices such as artificial hip joints and knee joints are hardened to improve wear resistance and hence to prolong the prosthesis lifetime.
Prosthetic knee and hip replacements can fail due to generation of wear debris from both metal and polymeric components. This in turn possibly induces a series of tissue reactions, including phagocytosis. Over a given time period, the chemical reactions cause osteolysis—an attack on the bone supporting the stem of the implant. Degradation of the bone tissue supporting the artificial joint eventually requires surgical intervention with inherent risk and cost. For more than a decade, cobalt chrome and titanium alloy prosthetic knee and hip replacements have been treated by ion implantation to increase their wear resistance, suppress osteolysis, and extend the effective component life. In this process, a beam of energetic nitrogen ions is directed at the surface of the implant, the ions penetrate and react with the metal, forming hard nitride precipitates which can increase the hardness, fatigue life, and water compatibility of the load bearing surface. Although ion implantation alters surface properties, it is not a coating process. As a result, there is no dimensional increase, change in surface finish, or risk of material delamination to the work-piece.
However, two important issues arise with this technique. First, although ion implantation shows some improvement on wear, the life spans of such prostheses are still limited (on average about 8-10 years) due to the inherent physical limitations of the technique as well as overall implant design limitations. The depth of modification produced by conventional nitrogen implantation is only 0.1-0.2 micrometers at typical energies of 50-100 keV. This implanted layer is believed to be insufficient for prosthetic applications, particularly knee joints, where high load conditions are applied along with frequent motion. When this layer is worn away, the wear rate can increase significantly. Second, the cost of beam-ion implantation can be quite high because of the limited size of an ion beam (often only a few square centimeters in area) and the line-of-sight geometry. In order to treat a number of devices, the ion beam has to be scanned to cover all the workpieces. This increases the required processing time, and therefore the cost. In addition, components must be manipulated in such a way that the ion beam “paints” the critical 3-dimensional wear surfaces evenly. This also requires a long duration of processing. Furthermore, complex fixtures are required to perform these manipulations. Because of the limitations described above, and the relatively high capital investment in implantation facilities the current cost of ion implantation for orthopedics can be quite high.